General References Baaie Conosion Course, Ed. NACE. Hautoto (1976). Boekria, J . O'M; Fkddy, A. K. N., Eds. Modem Electmchmistry; Plenum: New York, 197@Vol. 11. Evam, Uliek R. Tho Corrosion ond OxidationofMefola Scisntilic Plincipler and Roctieel Applicotionr:Arnold: Landan, 1971.
A Simple Demonstration of the Activation Enerav Concept S u E M m E D BY
Romeu C. Rocha-Fllho Unlveroldade Federal de Sio Carlos Calm Postal 676 560 Carlos-SP, 13560 Brarll CHECKEC BY
hnel Mosher M l ~ o o u r Soulhem l College Joplln, MO 64801 Activation energy is a concept that has different definitions in the various theoretical approaches to chemical kinetics, as has been clearly shown by Dacey (I). qhese different concepts of "activation energy" are related to each other hut are numerically distinct (I). Nevertheless, activation energy may always he visualized simply as an "energy" barrier that has to he surmounted for the reaction to proceed to ~roducts. Although activation energy is presented to students quite earlv. it often seems mvsterious and difficult to visualize. ~ h & ;the purpose of this note is to present a simple visual demonstration that has moved useful to h e l students ~ overcome this difficulty. This demonstration is based on the high coefficient of thermal expansion of organic liquid substances. Ethanol-a = 11.2 X 10 -4 K-1 @-was chosen to he used due to its commonness and low toxicity.
Equipment and Materials A glass apparatus such as the one shown in the figure provides an analogy for the typical "energy" vs. reaction coordinate plot for a spontaneous reaction ( 3 , 4 ) . The "reagents" and "products" flasks are interconnected by acapillary tube. A capillary tube is also placed above each flask so that pressure balance is always present. Ethanol (200mL),colored with a dye for visual enhancement, and a 250-mL heaker of hot water are all that is needed for the demonstration to be carried out.
Demonstration Procedure First fill the "reagents" flask with the colored ethanol up to a )pointwhere only a small bubble of air is left below the capillarytube above it. Before continuing, the students' attention should he called to the similarity that now exists between the demonstration setup and a c.ar's gasoline tank connected by rubber tubing to a vessel on the 'floor. Although the vessel is at a lower level of potential energy than the car's gasoline tank, the gasoline does not flow to the vessel. Why? Because there is a potential-energy barrier (the path of the rubber tubing) that has to he surmounted. Now, continue by immersing the "reagents" flask in hot water: provision of energy. Immediately the ethanol will start to expand slowlv as its tem~eraturerises: this wav. .. the ethanol will rise in the "energy" barrier capillary (as well as in the caprllary ahuve the "reagentr" flatkj. Once the ethanol reaches the t*,p of the tmrier it will be siphoned to the "products" flask: enough energy (amvation energy) has been provided for the harrier to he surmounted. This demonstration characterizes a better analogy for activation energy than one recently reported by Hansen (5),since energy is actually provided to the "reagents". Note: If no facilities for manipulation of glass are available, the apparatus far this demonstration may be assembled using Pyrex tubing for the "reagents" and "products" flasks, small connections of capillary tubing at the top and the bottom of the flasks, and Tygon tubing for making the connection. Nevertheless, the Tygon tubing used should be of small diameter; otherwise the volume available for expansion of the ethanol may become so large as to cause the demonstration not to work. Llterature Cited 1. D s w , P. D. J Chem Educ. 1981.58.612. 2. Castellan, G.W. Physical Chern&by,2nd ed.: Addiaon-Waley: Reading MA, 1971; p m 3. Ref2, p 181. 4. Atkins, P. W . Phwicol Chemktry, 2nd ed.;Freeman: San Frantiseo, 1982;~ 9 7 9 . 5. Hsnsan,R.C.J. ChemEduc. 1984,61,804. 0".
A Boiling Demonstration at Room Tempkrature' S U B M ~ BY D
Axel Hablch Rdrnlbuhl Kanl~l*~s~hde Rdrnlslrarse 54 CH-BOO1 Zurlch, Swltrerland CHECKED eY
George L. Gilbert Denlson Unlverrlty Granville, OH 43023
The apparatus used in the reaction-analagldemonsIration.
The dependence of the boiling point of a liquid on the attractive forces between the particles of which it is composed can be strikingly demonstrated simply by mixing two liquids able to form an azeotropicmixture with a sufficiently low minimum boiling point. In such mixtures, the attractive forces hetween the particles of different kinds are weaker than those between particles of the same species. Therefore, the particles can respond more easily to the urge of entropy and, taking up some potential energy, arrange themselves ai a greater distance from each other. This is manifested macroscopically hy a significant cooling during mixing, a more than additive increase in volume. and an increased volatilitv or a lowered boiling point of the' mixture. Evaporation has, so to speak, already occurred to some degree within the Volume 65
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liquid, thus greatly facilitating the transition of the particles from the liquid phase into the gas phase. Especially suitable for the demonstration are the common liquids methyl formate (formic acid methyl ester, bp 32'C) and isonentane (2-methvlbutane.. bn . 28'C). A mixture of 36% of kethyl formate and 64% of isopentane by volume boils a t 17°C'~2.that is. below room temuerature. Themixine ratio of the two liquids is not very critkd, and they may be mixed in either wav. However. the followine - procedure proved best. Place a boiling stone in a test tube, add 3.6 mL of methyl formate, and cover it with 6.4 mL of isopentane, which hada lower specific eravitv. Onlv some tiny air bubbles, originating from the b&ngstone, may possibly be seen a t thisstage. Now, close the test tube with a rubber stopper, invert i t twice, and remove the stopper. The mixture immediately starts to boil smoothly. As a "bonus", the dependence of the hoiling point on the gas pressure above the liquid can also easily be demonstrated. Just close the test tube with the rubber stopper, and hoiling will stop until the stopper is removed again (do not wait too long!). Larger volumes of methyl formate and isopentane may also he used. However, due to the strong cooling during mixing, hoiling of the mixture is delayed until the larger volume of liquid has taken up enough heat from the surroundings. Moreover, care has to be taken because of the larger vapor volume developed. If the room temperature is comparatively high and there-
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fore too close to the hoilingpoint of ispentane, the analogous demonstration can be performed with pentane (bp 36°C) instead. The corresponding azeotropic mixture is composed of 42% of methyl formate and 58%of pentane by volume and boils a t 22'C1. While it is difficult to explain why the molecular attraction between methyl formate and pentane or isopentane is weaker than that between molecules of the same kind, it is obvious that the greater effect observed with isopentane must orieinate in its branched structure. This structure leads to a-smaller molecular surface and keeps the molecules farther auart. . . botheffects being known to reduce the van der Waals forces. There are manv more azeotropic mixtures with a miniHowever, they all have their shortcommum hoiling ines. Either the components represent a greater health hazar& or the hoiling point of the mixture is too close to those of the pure components, or i t lies too far above room temperature. In the last case, of course, the components and their azeotropic mixture could he heated to the corresponding minimum hoiling point-hut this would detract from the elegance of the demonstration to a certain degree.
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'Horsley, L.H.. Compiler. Azeotropic Data-ill; Advances in Chemistry Series 116, Am. Chem. Soc.: Washington, DC, 1973; p 115. Weast, R. C..Ed. Handbook of Chemistry and Physics. 66th ed.: Chemical Rubber Co.: Cleveland. OH, 1985; p D-14.
